<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<script src="statics/js/common/jquery-1.4.2.min.js"></script>
<form>
    <input type="radio" class="ss" id="aa" checked>
</form>


<script>
var val = $("#aa:checked").length;
alert(val)
</script>
</body>
</html>